1. Field of the Invention
The present invention relates to piezoelectric accelerometers, and more particularly, to a signal conditioning circuit incorporating fast startup, low frequency response and micro power operation.
2. Description of the Prior Art
Piezoelectric sensors are used in many applications, including accelerometers and vibration sensors. Because the piezoelectric element (crystal) is an inherently high impedance device, it is often necessary to buffer or condition the signal generated by the piezoelectric element. A piezoelectric amplifier is an electronic circuit that converts the high impedance output of the crystal to a more usable, low impedance signal.
There are two general approaches used to construct piezoelectric amplifiers: transistor designs and op-amp designs. These are respectively depicted in FIGS. 1(a), 2(a), in block diagram form. In FIG. 2(a), the Voltage Reference sets the Bias Output Voltage (BOV) and provides the circuit with a known voltage point. The Piezo Crystal produces electrical output when induced with physical vibration. It has a capacitance that affects gain and the low-pass corner frequency. R1 also affects low-pass corner; R2 affects the high-pass corner: C1 affects both gain and the low-pass corner; and C2 affects both gain and the high-pass corner. The “corner” frequency is defined as the frequency at which the amplifier AC response varies by ±3 dB from nominal.
Because of the high impedance output of the piezoelectric element, the amplifiers in FIGS. 1(a) and 2(a) must have very high input impedance. High input impedance is achieved by using semiconductor devices (transistors or operational amplifiers) with very low input bias currents and by using large values of resistance, often in the gigaohm range, to bias the amplifier. The high resistances coupled with the capacitance of the piezoelectric element results in very large RC (resistor-capacitor) time constants, where the RC product is the charge time, in seconds. See, for instance, U.S. Pat. Nos. 2,857,462, 3,400,284, and 3,452,287.
The negative effects of op-amp saturation are well known within the industry, such as discussed in Tutorial MT-084 by Analog Devices Corporation, Copyright 2009, and Application Report SLOA067 by Texas Instruments Corporation, Section 4.1, Copyright September 2001. Traditional transistor based designs (FIG. 1(a)) have long startup times, as shown in FIG. 1(b), due to the slow charging of the PZT crystal capacitance through a very large value biasing resistor. Traditional op-amp based designs (FIG. 2(a)) have long startup times, as shown in FIG. 2(b), due to both the slow charging of the piezoelectric element through the large value feedback resistor (R2), and a saturation recovery delay due to the slow feedback's inability to close the loop during startup.
Startup time is an important parameter for many low power piezoelectric sensor applications employing battery power or energy harvesting. The startup time of a piezoelectric amplifier is generally defined as the time it takes from power-up to when the analog output signal has stabilized and the low frequency information is valid. The startup time is related to the bias output voltage (BOV) stabilization (or settling) time, since while the BOV is stabilizing, the low frequency information is contaminated.
The design of a vibration sensor that consumes only microamps of current with a high pass response in the sub Hertz range and a high dynamic range places many constraints on the piezoelectric amplifier design that are incompatible with a fast startup time. Piezoelectric amplifier designs traditionally have had the startup time coupled to the capacitance of the piezoelectric element and the high impedance of the amplifier. In prior art, the low frequency response is set by both the capacitance of the piezoelectric crystal and a large resistance. Since the capacitance of the crystal is small, approximately 1 nF, biasing resistor values have to be large, on the order of a few gigaohms, to place high pass corner frequency into sub-Hertz range. At power up, the capacitance of the piezoelectric sensing element along with other circuitry must be charged through this large resistance before the bias output voltage (BOV) can stabilize. In a traditional piezoelectric amplifier, it can take a few seconds for a sensor to startup and for the BOV to stabilize. For some applications, such as low power wireless sensor networks, this startup time is too long. See, for instance, Connection Technology Center Inc. Model AC 102 data sheet (settling time 2.5 seconds). Connection Technology Center Inc. Model AC 131 data sheet (settling time 2 seconds), and PCB® Piezotronics, Inc. Model 608A11 data sheet (settling time 2.0 seconds).
It will be appreciated that existing piezoelectric amplifier designs have the startup time of the amplifier closely coupled to the high-pass corner frequency. Traditional piezoelectric amplifiers achieve fast startup only by sacrificing low frequency response, low noise characteristics and/or micro power operation. In prior art, a low noise characteristic, micro power operation, sub-Hertz low frequency response and fast startup have been mutually exclusive (unattainable). See FIG. 8, which is a data sheet of a Connection Technology Center Inc. Model AC312, having Sub-Hertz response, micro power, low-noise, but slow startup (1 second). This disadvantage is especially great in low power piezoelectric sensor applications employing battery power or energy harvesting. In many of these applications, only microamps of average current is available and duty cycles (percentage of sensor on-time) are well below one percent. In these cases, the sensor and amplifier must be stabile within milliseconds after the “node” powers up. For instance, see “WirelessHART & Internet Protocol Wireless Sensor Networks Achieve Industry's Lowest Power Consumption at Less Than 50 μA per Node,” by Linear Technology Corporation, Oct. 16, 2012.